In the "Principles of Neural Science" 5th edition in the discussion about electrical synapses it says:
During excitatory synaptic transmission at an electrical synapse, voltage-gated ion channels in the presynaptic cell generate the current that depolarizes the postsynaptic cell. Thus these channels not only depolarize the presynaptic cell above the threshold for an action potential but also generate sufficient ionic current to produce a change in potential in the postsynaptic cell.
To generate such a large current, the presynaptic terminal must be big enough for its membrane to contain many ion channels. At the same time, the postsynaptic cell must be relatively small. This is because a small cell has a higher input resistance (Rin) than a large cell and, according to Ohm's law (V = I x Rin), undergoes a greater voltage change in response to a given presynaptic current (I).
I get that Ohm's law states this, but what is really going on? Why should the resistance encountered by charge as it is moving through gap-junctions from the presynaptic to the postsynaptic cell affect the voltage (electrical potential) across the membrane of the postsynaptic cell with its environment. Once the charge gets to the postsynaptic cell, it seems like it should no longer be affected by the resistance it encountered along the way.